The invention relates to a continuous-flow steam generator having a combustion chamber for fossil fuel which is followed on the fuel-gas side, via a horizontal gas flue, by a vertical gas flue, the containment walls of the combustion chamber being formed from vertically arranged evaporator tubes welded to one another in a gastight manner.
In a power plant with a steam generator, the energy content of a fuel is utilized for the evaporation of a flow medium in the steam generator. In this case, the flow medium is conventionally carried in an evaporator circuit. The steam supplied by the steam generator may, in turn, be provided, for example, for driving a steam turbine and/or for a connected external process. When the steam drives a steam turbine, a generator or a working machine is normally operated via the turbine shaft of the steam turbine. Where a generator is concerned, the current generated by the generator may be provided for feeding into an interconnected and/or island network.
The steam generator may in this case be designed as a continuous-flow steam generator. A continuous-flow steam generator is known from the paper xe2x80x9cVerdampferkonzepte fxc3xcr Benson-Dampferzeugerxe2x80x9d [xe2x80x9cEvaporator concepts for Benson steam generatorsxe2x80x9d] by J. Franke, W. Kxc3x6hler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), number 4, p. 352-360. In a continuous-flow steam generator, the heating of steam generator tubes provided as evaporator tubes leads to an evaporation of the flow medium in the steam generator tubes in a single pass.
Continuous-flow steam generators are conventionally designed with a combustion chamber in a vertical form of construction. This means that the combustion chamber is designed for the heating medium or fuel gas to flow through it in approximately vertical direction. In this case, the combustion chamber may be followed on the fuel-gas side by a horizontal gas flue, a deflection of the fuel-gas stream into an approximately horizontal direction of flow taking place at the transition from the combustion chamber into the horizontal gas flue. However, because of the thermally induced changes in length of the combustion chamber, combustion chambers of this type generally require a framework on which the combustion chamber is suspended. This necessitates a considerable technical outlay in terms of the manufacture and assembly of the continuous-flow steam generator. The higher the outlay, the greater the overall height of the continuous-flow steam generator. This applies particularly in the case of continuous-flow steam generators which are designed for a steam power output of more than 80 kg/s under full load.
A continuous-flow steam generator is not subject to any pressure limitation, so that fresh-steam pressures well above the critical pressure of water (pcri=221 bar), where there is only a slight difference in density between a liquid-like and a vapor-like medium, are possible. A high fresh-steam pressure is conducive to high thermal efficiency and therefore to low CO2 emissions for a fossil-fired power station which, for example, can be fired with hard coal or else with lignite in solid form as fuel.
A particular problem is presented by the design of the containment wall of the gas flue or combustion chamber of the continuous-flow steam generator in terms of the tube-wall or material temperatures which occur there. In the subcritical pressure range to about 200 bar, the temperature of the containment wall of the combustion chamber is determined essentially by the height of the saturation temperature of water when wetting of the inner surface of the evaporator tubes can be ensured. This is achieved, for example, using evaporator tubes which have a surface structure on their inside. In this respect, in particular, internally ribbed evaporator tubes come under consideration, of which the use in a continuous-flow steam generator is known, for example, from the abovementioned paper. These so-called ribbed tubes, that is to say tubes with a ribbed inner surface, have particularly good heat transmission from the tube inner wall to the flow medium.
Experience has shown that it is not possible to avoid the situation where, when the continuous-flow steam generator is in operation, thermal stresses occur between adjacent tube walls of different temperature when these are welded to one another. This is the case, in particular, with regard to the portion of the combustion chamber connecting the latter to the horizontal gas flue following it, that is to say between the evaporator tubes of the exit region of the combustion chamber and the steam generator tubes of the entry region of the horizontal gas flue. These thermal stresses may markedly reduce the useful life of the continuous-flow steam generator and, in an extreme case, even give rise to tube fractures.
An object on which the invention is based is, therefore, to specify a fossil-fired continuous-flow steam generator of the abovementioned type which requires a particularly low outlay in terms of manufacture and assembly. Moreover, preferably a generator during the operation of which temperature differences at the connection of the combustion chamber to the horizontal gas flue following the latter are kept low. This is to be the case, in particular, for the mutually directly or indirectly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue following the combustion chamber.
This object and/or other objects are achieved, according to the invention, in that the continuous-flow steam generator has a combustion chamber with a number of burners arranged level with the horizontal gas flue. A plurality of the evaporator tubes are preferably capable of being acted upon in each case in parallel by flow medium. Further, in the exit region of the combustion chamber, a number of the evaporator tubes are preferably capable of being acted upon in parallel by flow medium being led through the combustion chamber before their entry into the respective containment wall of the combustion chamber.
The invention proceeds from the notion that a continuous-flow steam generator capable of being produced at a particularly low outlay in terms of manufacture and assembly, should have a suspension structure capable of being executed by simple means. A framework to be produced at comparatively low outlay in technical terms and intended for suspending the combustion chamber may, in this case, be accompanied by a particularly small overall height of the continuous-flow steam generator. A particularly small overall height of the continuous-flow steam generator can be achieved by the combustion chamber being designed in a horizontal form of construction. For this purpose, the burners are arranged level with the horizontal gas flue in the combustion chamber wall. Thus, when the continuous-flow steam generator is in operation, the fuel gas flows through the combustion chamber in an approximately horizontal main direction of flow.
Moreover, when the continuous-flow steam generator with the horizontal combustion chamber is in operation, temperature differences should be particularly low at the connection of the combustion chamber to the horizontal gas flue, in order reliably to avoid premature material fatigues as a result of thermal stresses. These temperature differences should be especially low, in particular, between mutually directly or indirectly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue, so that material fatigues as a result of thermal stresses are prevented particularly reliably in the exit region of the combustion chamber and in the entry region of the horizontal gas flue.
However, when the continuous-flow steam generator is in operation, that entry portion of the evaporator tubes which is acted upon by flow medium has a comparatively lower temperature than the entry portion of the steam generator tubes of the horizontal gas flue following the combustion chamber. To be precise, comparatively cold flow medium enters the evaporator tubes,in contrast to the hot flow medium which enters the steam generator tubes of the horizontal gas flue. Hence, when the continuous-flow steam generator is in operation, the evaporator tubes in the entry portion are colder than the steam generator tubes in the entry portion of the horizontal gas flue. Material fatigues as a result of thermal stresses are therefore to be expected at the connection between the combustion chamber and the horizontal gas flue.
If, however, preheated flow medium, instead of cold, enters the evaporator tubes of the combustion chamber, the temperature difference between the entry portion of the evaporator tubes and the entry portion of the steam generator tubes will also no longer be as great as would be the case if cold flow medium were to enter the evaporator tubes. The temperature difference can be reduced even further if the tube in which the preheating of the flow medium takes place by heating is connected directly to or else forms part of the evaporator tube connected indirectly or directly to the steam generator tubes of the horizontal gas flue. For this purpose, a number of the evaporator tubes are led through the combustion chamber before their entry into the containment wall of the combustion chamber. At the same time, this number of evaporator tubes are assigned to a plurality of evaporator tubes capable of being acted upon in parallel by flow medium.
The side walls of the horizontal gas flue and/or of the vertical gas flue are advantageously formed from vertically arranged steam generator tubes welded to one another in a gastight manner and capable of being acted upon in each case in parallel by flow medium.
Advantageously, in each case, a number of parallel-connected evaporator tubes of the combustion chamber are preceded by a common entry header system and followed by a common exit header system for flow medium. To be precise, a continuous-flow steam generator designed in this configuration makes it possible to have reliable pressure compensation between a number of evaporator tubes capable of being acted upon in parallel by flow medium, so that, in each case, all the parallel-connected evaporator tubes between the entry header system and the exit header system have the same overall pressure loss. This means that the throughput must rise in the case of an evaporator tube heated to a greater extent, as compared with an evaporator tube heated to a lesser extent. This also applies to the steam generator tubes of the horizontal gas flue or of the vertical gas flue which are capable of being acted upon in parallel by a flow medium and which are advantageously preceded by a common entry header system for flow medium and followed by a common exit header system for flow medium.
The evaporator tubes of the end wall of the combustion chamber are advantageously capable of being acted upon in parallel by flow medium and precede on the flow-medium side the evaporator tubes of the containment walls which form the side walls of the combustion chamber. This ensures a particularly beneficial cooling of the highly heated end wall of the combustion chamber.
In a further advantageous embodiment of the invention, the tube inside diameter of a number of the evaporator tubes of the combustion chamber is preferably selected as a function of the respective position of the evaporator tube in the combustion chamber. Evaporator tubes in the combustion chamber can thereby be adapted to a heating profile capable of being predetermined on the fuel-gas side. As a result of the influence brought about thereby on the throughflow of the evaporator tubes, temperature differences of the flow medium at the exit from the evaporator tubes of the combustion chamber are kept particularly low in a particularly reliable way.
For particularly good transmission of heat from the heat of the combustion chamber to the flow medium carried in the evaporator tubes a number of the evaporator tubes preferably and advantageously have on their inside, in each case, ribs forming a multiflight thread. In this case, advantageously, a pitch angle xcex1 between a plane perpendicular to the tube axis and the flanks of the ribs arranged on the tube inside is preferably smaller than about 60xc2x0, even more preferably smaller than about 55xc2x0.
To be precise, in a heated evaporator tube designed as an evaporator tube without internal ribbing, a so-called smooth tube, from a specific steam content on, the wetting of the tube wall necessary for a particularly good heat transmission can no longer be maintained. In the absence of wetting, the tube wall may be dry in places. The transition to a dry wall of this kind leads to what is known as a heat transmission crisis with an impaired heat transmission behavior. Therefore, in general, the tube wall temperatures at this point rise particularly sharply.
In an internally ribbed evaporator tube, however, as compared with a smooth tube, this heat transmission crisis arises only at a steam mass content of  greater than 0.9, that is to say just before the end of the evaporation. This is attributable to the swirl which the spiral ribs impart to the flow. On account of the difference in centrifugal force, the water fraction is separated from the steam fraction and is transported to the tube wall. The wetting of the tube wall is thereby maintained up to high steam contents, so that there are even high flow velocities at the location of the heat transmission crisis. This gives rise, despite the heat transmission crisis, to a relatively good heat transmission and, as a result, to low tube wall temperatures.
A number of the evaporator tubes of the combustion chamber advantageously have means for reducing the throughflow of the flow medium. In this case, it proves particularly beneficial if the means are preferably designed as throttle devices. Throttle devices may be, for example, fittings which are installed in the evaporator tubes and which reduce the tube inside diameter at a point within the respective evaporator tube. It also proves advantageous, in this case, to have means for reducing the throughflow in a line system which comprises a plurality of parallel lines and through which flow medium can be supplied to the evaporator tubes of the combustion chamber. At the same time, the line system may also precede an entry header system of evaporator tubes capable of being acted upon in parallel by flow medium. In this case, for example, throttle accouterments may be provided in one line or in a plurality of lines of the line system. By such means for reducing the throughflow of the flow medium through the evaporator tubes, it is possible for the throughput of the flow medium through individual evaporator tubes to be adapted to their respective heating in the combustion chamber. Temperature differences of the flow medium at the exit of the evaporator tubes are thereby additionally kept particularly low in a particularly reliable way.
Adjacent evaporator or steam generator tubes are preferably welded to one another in a gastight manner on their longitudinal sides advantageously via metal bands, so-called fins. These fins may even be connected firmly to the tubes during the process for producing the tubes and form a unit with these. The unit formed from a tube and fins is also designated as a finned tube. The fin width influences the introduction of heat into the evaporator or steam generator tubes. The fin width is therefore adapted, preferably as a function of the position of the respective evaporator or steam generator tubes in the continuous-flow steam generator, to a heating profile capable of being predetermined on the fuel-gas side.
In this case, a typical heating profile determined from experimental values or else a rough estimation, such as, for example, a stepped heating profile, may be predetermined as heating profile. By virtue of the suitably selected fin widths, even when different evaporator or steam generator tubes are heated in a widely differing way an introduction of heat into all the evaporator or steam generator tubes can be achieved in such a way that temperature differences of the flow medium at the exit from the evaporator or steam generator tubes are kept particularly low. Premature material fatigue as a result of thermal stresses are reliably prevented in this way. The continuous-flow steam generator consequently has particularly long useful life.
The horizontal gas flue preferably and advantageously has arranged in it a number of superheater heating surfaces which are arranged approximately perpendicularly to the main direction of flow of the fuel gas and the tubes of which are connected in parallel for a throughflow of the flow medium. These superheater heating surfaces, arranged in a suspended form of construction and also designated as bulkhead heating surfaces, are preferably heated predominantly convectively and follow the evaporator tubes of the combustion chamber on the flow-medium side. A particularly beneficial utilization of the fuel-gas heat is thereby ensured.
The vertical gas flue preferably and advantageously has a number of convection heating surfaces which are formed from tubes arranged approximately perpendicularly to the main direction of flow of the fuel gas. These tubes of a convection heating surface are connected in parallel for a throughflow of the flow medium. These convection heating surfaces, too, are preferably heated predominantly convectively.
In order, furthermore, to ensure a particularly full utilization of the heat of the fuel gas, the vertical gas flue advantageously has an economizer.
Advantageously, the burners are preferably arranged on the end wall of the combustion chamber, that is to say on that side wall of the combustion chamber which is located opposite the outflow orifice to the horizontal gas flue. A continuous-flow steam generator designed in this way can be adapted particularly simply to the burnup length of the fossil fuel. By the burnup length of the fossil fuel is to be meant, in this context, the fuel-gas velocity in the horizontal direction at a specific average fuel-gas temperature, multiplied by the burnup time tA of the flame of the fossil fuel. In this case, the maximum burnup length for the respective continuous-flow steam generator is obtained when the continuous-flow steam generator is under full load with the steam power output M, the so-called full-load mode. The burnup time tA of the flame of the fossil fuel is, in turn, the time which, for example, a coaldust grain of average size requires in order to burnup completely at a specific average fuel-gas temperature.
In order to keep material damage and undesirable contamination of the horizontal gas flue particularly low, for example due to the introduction of high-temperature molten ash, the length of the combustion chamber, defined by the distance from the end wall to the entry region of the horizontal gas flue, is preferably and advantageously at least equal to the burnup length of the fossil fuel in the full-load mode of the continuous-flow steam generator. This horizontal length of the combustion chamber will generally amount to at least 80% of the height of the combustion chamber, measured from the funnel lop edge, when the lower region of the combustion chamber has a funnel-shaped design, to the combustion chamber ceiling.
For a particularly beneficial utilization of the combustion heat of the fossil fuel, the length L (given in m) of the combustion chamber is preferably and advantageously selected as a function of the steam power output M (given in kg/s) of the continuous-flow steam generator under full load, the burnup time tA (given in s) of the flame of the fossil fuel and the exit temperature TBRK (given in xc2x0C.) of the fuel gas from the combustion chamber. In this case, with a given steam power output M of the continuous-flow steam generator under full load, approximately the higher value of the two functions (I) and (II) applies to the length L of the combustion chamber:
By xe2x80x9capproximatelyxe2x80x9d is to be meant, in this context, a permissible deviation in the length L of the combustion chamber of +20%/xe2x88x9210% from the value defined by the respective function.
The lower region of the combustion chamber is preferably and advantageously designed as a funnel. In this way, when the continuous-flow steam generator is in operation, ash occurring during the combustion of the fossil fuel can be discharged particularly simply, for example into an ash removal device arranged below the funnel. The fossil fuel may in this case may be coal in solid form.
The advantages achieved by means of the invention include but are not limited to the following. By virtue of some evaporator tubes being led through the combustion chamber before their entry into the containment wall of the combustion chamber, temperature differences in the immediate vicinity of the connection of the combustion chamber to the horizontal gas flue are particularly low when the continuous-flow steam generator is in operation. Consequently, when the continuous-flow steam generator is in operation, the thermal stresses at the connection of the combustion chamber to the horizontal gas flue, which are caused by temperature differences between directly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue, remain well below the values at which, for example, there is the risk of tube fractures. The use of a horizontal combustion chamber in a continuous-flow steam generator is therefore possible, even with a comparatively long useful life. Moreover, designing the combustion chamber for an approximately horizontal main direction of flow of the fuel gas affords a particularly compact form of construction of the continuous-flow steam generator.
This, makes it possible, when the continuous-flow steam generator is incorporated into a power station with a steam turbine, to also have particularly short connecting tubes from the continuous-flow steam generator to the steam turbine.